1. Field of the Invention
A certain aspect of the present invention relates to a fusing device and an image forming apparatus including the fusing device.
2. Description of the Related Art
A fusing roller of an image forming apparatus such as a copier or a printer typically includes a heat generator such as a halogen heater and is pressed against a pressing roller to form a nip (fusing nip). Such a fusing roller fuses a toner image onto a recording sheet (may simply be called a sheet) fed into the nip by the pressure at the nip and the radiant heat from the heat generator.
In the field of high-speed, low power-consumption image forming apparatuses, there is a trend to make a fusing roller thinner to be able to promptly raise the temperature of the fusing roller and thereby to smoothly fuse a toner image onto a sheet. Meanwhile, patent document 1 discloses a method for reducing the heat-up time of a heating part (fusing roller) by supplementarily supplying power to a heat generator (capacitor heater) of the heating part from an auxiliary power supply (capacitor). Also, patent document 2 discloses a method for improving the power efficiency by supplementarily supplying power to a heat generator of a heating part from an auxiliary power supply while a sheet is being passed through a fusing nip.
Further, there is a known technology for fusing a toner image onto a sheet where a rod-like central light distribution AC heater (center heater) having a light-emitting part in the center and a rod-like edge light distribution AC heater (edge heater) having light-emitting parts at the corresponding ends are selectively used according to the size of the sheet.
With a thin fusing roller, the temperature distribution in the center and edge portions of the roller tends to become non-uniform and the temperature rise rate of the edge portions of the roller tends to become slower than that of the center portion during warm-up. Also, since the heat capacity of such a thin fusing roller is small, the temperature of the edge portions of the roller falls significantly when fusing a toner image onto a sheet immediately after start-up. To reduce or solve this problem, an auxiliary heater (an AC halogen heater or a capacitor heater) having an edge light distribution or a flat light distribution (having a heat generator that covers the entire length of the heater) is used during warm-up or a fusing process.
However, when small-size sheets (e.g., A5-size sheets) are fed consecutively into the fusing nip (when small sheets are processed consecutively), the temperature of a non-fusing portion (non-sheet-passing portion that is not in contact with the sheets) in the width direction of the fusing roller rises significantly, the temperature of a fusing portion (sheet-passing portion that is in contact with the sheets) falls significantly, and as a result the fusing performance is reduced. Also, when small-size sheets are fed consecutively into a fusing nip or during a next print job after consecutive feeding of small-size sheets, the amount of remaining electric energy in the auxiliary power supply (capacitor) may decrease and the auxiliary power supply may become unable to supply power. This in turn causes an unbalanced temperature distribution in the width direction of the fusing roller and reduces the fusing performance.    [Patent document 1] Japanese Patent Application Publication No. 2000-315567    [Patent document 2] Japanese Patent Application Publication No. 2005-216784